The present invention relates to a monitoring of a state of oil-filled transformers using information such as transformer oil temperature, current and ambient temperature.
A stable supply of electricity is important and it is therefore necessary to periodically monitor transformers to forestall possible failures. For this purpose, monitoring a voltage, current and oil temperature of the transformers is effective. The monitoring of the oil temperature generally involves estimating an oil temperature from an average load factor, visually checking a temperature reading on a thermometer, comparing the estimated oil temperature with the temperature reading on the thermometer to decide whether the transformer in question is normal or not, and at the same time checking if the temperature reading on the thermometer has exceeded a predetermined value.
Another practice involves the use of a thermometer with an alarm contact which issues an alarm for detection of an anomaly when the oil temperature exceeds a set temperature.
Another method of monitoring is disclosed, for instance, in JP-A-5-227644 which, when determining a remaining service life from a maximum winding temperature, calculates an oil temperature rise and determines an oil temperature by adding a measured ambient temperature to the oil temperature rise.
In another method disclosed in JP-A-8-213246, a means is provided that calculates a transformer oil temperature from a reading on a thermometer attached to the outside of a case of the oil-filled transformer and from a measured load factor of the transformer.
In the above conventional method using a visual check, however, since an oil temperature change lags load factor and ambient temperature changes, checking the measured temperature after the load factor or ambient temperature has changed cannot easily determine if it is a proper temperature or not.
Further, JIS C 4304 specifies that a limit of an oil temperature rise is 50K at maximum during the rated load operation and that the transformer can be used in an ambient temperature range of between −20° C. and 40° C. Thus, to precisely estimate the oil temperature requires considering influences of ambient temperature.
When the oil temperature exceeds the set temperature of the thermometer with an alarm contact, an alarm is issued for detection of an anomaly. However, since the oil temperature is a sum of a temperature rise caused by a transformer loss and a temperature rise caused by the ambient temperature, if the ambient temperature is low, the oil temperature will not reach the set temperature even during an overload operation. Further, even when the transformer is used below the rated load, an alarm may be issued if the ambient temperature is high and the set temperature is exceeded. This method therefore could not decide whether the oil temperature is correct or not.
Nor can the conventional monitoring system determine if the oil temperature is correct or not.
Further, since the above conventional technique uses a measured value for the ambient temperature in determining the maximum winding temperature, the fact is not taken into account that an oil temperature change lags behind an ambient temperature change.
Further, since the conventional technique does not consider ambient temperature variations when indirectly measuring the oil temperature from the case outer surface temperature, an error occurs between the measured value and the converted value if the ambient temperature changes sharply. Moreover, when the converted value is based on a case measured value, which indirectly measures the oil temperature, since the measured value constitutes a reference, it is not possible to estimate accurately what the true oil temperature is.
Therefore, if an abnormal condition occurs in connected portions of circuit within coil and case as a result of progressive degradations over time or vibrations caused by earthquakes, oil is heated and its temperature rises. However, since the true oil temperature cannot be estimated accurately, it is not possible to decide that an abnormal condition has occurred inside the transformer.